Usage

accept: to agree to	except: to exclude
adapt: to adjust or become accustomed	adopt:to accept or take as one's own
affect: to pretend, to influence	effect: the result of a cause
all ready: completely prepared	already: "before now"
all together: as a group	altogether: completely
a lot: a large amount	allot: to assign a part or portion
altar: a small raised structure used for religious purposes	alter: to change
can: is able	may: is allowed or permitted
capital: city where a center of government is located	capitol: important building within a capital city
complement: add to in a positive manner	compliment: express respect for or admiration
complementary: serving to complete	complimentary: free
convince: to satisfy by reason or argument	persuade: to bring about
farther: refers to physical distance	further: refers to extent or degree
fewer: deals with actual numbers	less: deals with a nonspecific quantity
lay: to place ("lay it on the table")	lie: to recline("lie down on the bed")
principal: person, place, or thing that is first in importance	principle: a fundamental truth or force

Also be careful not to confuse homonyms such as there, their, and they're or too, to, and two

2. Politically Correct Terms

Outdated Term	Correct Term
fireman/firewoman	firefighter
chairman/chairwoman	chairperson, chair
housewife	homemaker
mankind	humankind, humanity
postman, mailman	postal worker, mail carrier
salesman	sales representative, salesperson
spokesman	representative, spokesperson
businessman/businesswoman	businessperson
policeman/policewoman	police officer
waitress	waiter, wait staff
actress	actor